The present application relates to a current-efficient device and method for reducing the concentration of matrix ions of opposite charge to ions to be analyzed, and specifically for use of an ion chromatography suppressor or to a pretreatment device.
Ion chromatography is a known technique for the analysis of ions which typically includes a chromatographic separation stage using an eluent containing an electrolyte, and an eluent suppression stage, followed by detection, typically by an electrical conductivity detector. In the chromatographic separation stage, ions of an injected sample are eluted through a separation column using an electrolyte as the eluent. In the suppression stage, electrical conductivity of the electrolyte is suppressed but not that of the separated ions so that the latter may be determined by a conductivity cell. This technique is described in detail in U.S. Pat. Nos. 3,897,213; 3,920,397; 3,925,019; and 3,926,559.
Suppression or stripping of the electrolyte is described in the above prior art references by an ion exchange resin bed. A different form of suppressor column is described and published in U.S. Pat. No. 4,474,664, in which a charged ion exchange membrane in the form of a fiber or sheet is used in place of the resin bed. The sample and eluent are passed on one side of the membrane with a flowing regenerant on the other side, the membrane partitioning the regenerant from the effluent of chromatographic separation. The membrane passes ions of the same charge as the exchangeable ions of the membrane to convert the electrolyte of the eluent to weakly ionized form, followed by detection of the ions.
Another membrane suppressor device is disclosed in U.S. Pat. No. 4,751,004. There, a hollow fiber suppressor is packed with polymer beads to reduce band spreading. There is a suggestion that such packing may be used with other membrane forms. Furthermore, there is a suggestion that the function of the fiber suppressor is improved by using ion exchange packing beads. No theory is set forth as to why such particles would function in an improved manner.
Another suppression system is disclosed in U.S. Pat. No. 4,459,357. There, the effluent from a chromatographic column is passed through an open flow channel defined by flat membranes on both sides of the channel. On the opposite sides of both membranes are open channels through which regenerant solution is passed. As with the fiber suppressor, the flat membranes pass ions of the same charge as the exchangeable ions of the membrane. An electric field is passed between electrodes on opposite sides of the effluent channel to increase the mobility of the ion exchange. One problem with this electrodialytic membrane suppressor system is that very high voltages (50-500 volts DC) are required. As the liquid stream becomes deionized, electrical resistance increases, resulting in substantial heat production. Such heat is detrimental to effective detection because it greatly increases noise and decreases sensitivity.
In U.S. Pat. No. 4,403,039, another form of electrodialytic suppressor is disclosed in which the ion exchange membranes are in the form of concentric tubes. One of the electrodes is at the center of the innermost tube. One problem with this form of suppressor is limited exchange capacity. Although the electrical field enhances ion mobility, the device is still dependent on diffusion of ions in the bulk solution to the membrane.
Another form of suppressor is described in U.S. Pat. No. 4,999,098. In this apparatus, the suppressor includes at least one regenerant compartment and one chromatographic effluent compartment separated by an ion exchange membrane sheet. The sheet allows transmembrane passage of ions of the same charge as its exchangeable ions. Ion exchange screens are used in the regenerant and effluent compartments. Flow from the effluent compartment is directed to a detector, such as an electrical conductivity detector, for detecting the resolved ionic species. The screens provide ion exchange sites and serve to provide site-to-site transfer paths across the effluent flow channel so that suppression capacity is no longer limited by diffusion of ions in the bulk solution to the membrane. A sandwich suppressor is also disclosed including a second membrane sheet opposite to the first membrane sheet and defining a second regenerant compartment. Spaced electrodes are disclosed in communication with both regenerant chambers along the length of the suppressor. By applying an electrical potential across the electrodes, there is an increase in the suppression capacity of the device. The patent discloses a typical regenerant solution (acid or base) flowing in the regenerant flow channels and supplied from a regenerant delivery source. In a typical anion analysis system, sodium hydroxide is the electrolyte developing reagent and sulfuric acid is the regenerant. The patent also discloses the possibility of using water to replace the regenerant solution in the electrodialytic mode.
U.S. Pat. No. 5,045,204 discloses an electrodialytic device using an ion exchange membrane separating two flowing solutions in flow-through channels for generating a high purity chromatography eluent (e.g., NaOH). Water is electrolyzed in a product channel to provide the source of hydroxide ion for sodium which diffuses across the membrane. The patent discloses a mode of eliminating hydrogen gas generated in the product channel.
U.S. Pat. No. 5,248,426 discloses a suppressor of the general type described in U.S. Pat. No. 4,999,098 in an ion chromatography system in which the effluent from the detector is recycled to the flow channel(s) in the suppressor adjacent the sample stream flow channel.
U.S. Pat. No. 5,597,481 disclosed a suppressor-type device of the foregoing type used in sample pretreatment to reduce or suppress matrix ions in the eluent of opposite charge to the analyte ions and then to analyze the analytes in their conductive forms. Using existing suppressor devices, ion exchange interactions and hydrophobic interaction of the analyte, particularly in the eluent flow channel, affects recovery of certain analytes such as oligonucleotides and oligosaccharides. In order to improve recovery, high concentrations of eluents coupled with solvents are generally used. Similarly, in order to elute certain highly charged multifunctional analytes from the chromatographic column, high concentrations of eluents are normally used. High concentrations of eluents, however, are not easily suppressed.
In all of the disclosed approaches, currents higher than theoretically predicted are required for achieving quantitative suppression. Under high eluent concentration conditions, this high current translates into heat generation and high background noise. Therefore, there is a need for a suppressor that would enable suppression of a wide range of eluent concentration and operate near the current-efficient faradaic regime.
There is a need to increase the current efficiency of suppressors and suppressor-like pretreatment devices to permit suppression of a high concentration of eluent without the detrimental effects of excess heat generation. Similarly, in sample preparation applications it would be useful to have a suppressor that would enable good recovery of analytes and suppress high concentrations of eluent or mobile phase.